Add Contributor Licence Agreement auto signing

[lazyoracle]

Is your feature request related to a problem? Please describe.
Currently there is no way to ascertain that new (external?) contributors to the codebase don't have any conflicts (eg with their current employers) and properly ascertain code ownership/licensing

Describe the solution you'd like
Every PR from a new contributor should automatically trigger a digital signing of the CLA

Describe alternatives you've considered
Manually require every external contributor to provide an email (otherwise written) confirmation clarifying no conflicts

Additional context
Suggested solution: cla-bot